The present invention relates to a method for the bonding of polyimide film, and in particular to a method of improving the bond strength of polyimide film bonding to a metal sheet, e.g. a copper foil.
Some kinds of synthetic polymers are used as a substrate in the production of printed electronic circuits. Since polyimide has the characteristics of good thermal stability, low dielectric constant, chemical resistance and easy processing, it is widely used in the production of electronic packaging materials, such as multichip module, flexible printed circuit boards and tape automatic bonding, etc. In addition, polyimide can improve the signal propagation and reduce the crosstalk of the resulting product when it is bonded to a metal foil. Although polyimide material has the above advantages, the bond strength between the polyimide substrate and a metal foil is poor. Therefore, it is important for a technique to be developed to increase the bond strength of polyimide substrate when it is bonded to metal foils.
Conventionally, a polyimide film is bonded to a metal foil, usually a copper foil, for use in the production of printed electronic circuits. In order to improve the bond strength of the polyimide film, the copper foil is electroplated in a copper plated solution, wherein the copper foil is used as a cathode so that a rough layer of copper is formed thereon. This rough layer can increase the bond strength between the copper foil and the polyimide film, but the deposited copper easily develops into dendrite even after etching, and therefore the quality of the product is affected.
A method for improving the bonding between copper foil and polymer substrate is disclosed in U.S. Pat. No. 4,010,005, in which a copper foil is electroplated in a plating solution which contains a suspension of insoluble fine particles of epoxy resin, silica powder and barium sulfate, and a metal containing plate of insoluble fine particles is formed thereon. The copper foil thus prepared is bonded to epoxy resin-impregnated glass cloth. It is found that the bond strength has been increased. The bond strength of the copper foil prepared by this method when bonding to a polyimide substrate is not mentioned in said U.S. Patent.
However, it is found that the copper foil prepared by the method disclosed in said U.S. Patent is not suitable for an electroplation in a metal plated solution with a high temperature, e.g. above 50.degree. C., because the plating solution contains epoxy resins which have a low softening point and will soften in the heated metal plated solution to form a film to resist the deposition of metal. In addition, the copper foil prepared from the epoxy resin containing plating solution can not be used for bonding on a polyimide substrate, because the resulting product usually will be subject to a subsequent process involving temperatures greater than 180.degree. C.
In addition, the adhesives which were available for the bonding of metal foils, particularly copper foils, to polyimide substrate were not heat-resistant enough, and resulted in poor peeling resistance of the bonding.